Basis of Presentation (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Retained Earnings Adjustments [Line Items]
Decrease in net income attributable to common stockholders	$ 59.5	$ (48.3)	$ (241.0)	$ (216.5)
Decrease in income attributable to common stockholders, pre-tax	24.6	$ (75.8)	(375.7)	$ (290.6)
Net over-capitalization of deferred acquisition costs
Retained Earnings Adjustments [Line Items]
Decrease in net income attributable to common stockholders	9.9		9.9
Decrease in income attributable to common stockholders, pre-tax	$ 14.2		$ 14.2
Net over-capitalization period	10 years